1. Field of the Invention
The present invention relates to an EEPROM memory cell with high immunity to programming errors.
2. Description of the Prior Art
As is known, one of the main problems in EEPROM memory cells is the breakdown of the cell's tunnel oxide. This type of breakdown is random and unpredictable, and therefore reduces the performance of this device, limiting to approximately 10,000 the number of assured cycles of the cell during the life of the circuit in which it is inserted. This limitation in the number of write cycles reduces the field of applicability of this type of device.
In order to overcome the above limitation, it has been suggested to produce EEPROM memories with "100% redundancy" cells, i.e. with cells each of which is formed by two storage elements arranged in parallel, with the same bit written to both elements, so that the cell can be read or written correctly even if one of the elements has broken down. In this manner the probability of a random breakdown occurring in both tunnel oxides of the two storage elements which constitute the EEPROM memory cell becomes extremely low, and the maximum number of programming cycles per cell consequently increases. The breakdown events of the two elements are in fact separate and random, and the probabilities that each might occur are consequently independent: therefore the probability of random breakdown of the entire cell is given by the product of the two probabilities of the individual events.
These EEPROM memories are obviously read by using a cell state sensing element which is capable of sensing correctly even if one of the storage elements has broken down. Said "redundant" cells, besides being readable, should however be writable and erasable without errors when one of the two storage elements, usually the tunnel oxide, has broken down. This second requirement is only partially met in known redundant EEPROM memories due to interferences between neighboring cells which will be explained hereinafter.
The aim of the invention is now to provide a redundant EEPROM memory cell of the above described type wherein not only reading but also writing and erasing can always occur without errors even in the case of breakdown of one of the storage elements.